Generally, the manufacturing process of a semiconductor device includes forming active regions in a semiconductor substrate, depositing and patterning various insulating, conductive, and semiconductor layers over the substrate in sequential steps. In the semiconductor device, field effect transistors (FETs) are widely used for switching, amplification, filtering, and other tasks related to both analog and digital electrical signals. Most common among these are metal-oxide-semiconductor field effect transistors (MOSFET or MOS), in which a gate stack is energized to create an electric field in an underlying channel region of the semiconductor substrate, by which electrons are allowed to travel through the channel between a source region and a drain region of the semiconductor substrate.
The gate stack includes gate dielectric layers as an electrical isolation between the gate electrode and the semiconductor substrate. With the reduction in the feature sizes of the semiconductor device, the reliability of the gate stack becomes increasingly more dependent on the insulating property of the gate dielectric layers. The gate leakage current may increase with the reduced thickness of the gate dielectric layers. Challenges are encountered during conventional processing of high-performance semiconductor devices with a reduced feature size. Accordingly, improvements in semiconductor devices and methods for fabricating the same are to be sought.